Data input devices, such as computer mice, touch screens, trackballs and the like, are well known for inputting data into and interfacing with personal computers and workstations. Such devices allow rapid relocation of a cursor on a monitor, and are useful in many text, database and graphical programs. A user controls the cursor, for example, by moving the mouse over a surface to move the cursor in a direction and over distance proportional to the movement of the mouse.
Computer mice come in both optical and mechanical versions. Mechanical mice typically use a rotating ball to detect motion, and a pair of shaft encoders in contact with the ball to produce a digital signal used by the computer to move the cursor. One problem with mechanical mice is that they are prone to inaccuracy and malfunction after sustained use due to dirt accumulation, etc. In addition, the movement and resultant wear of the mechanical elements, particularly the shaft encoders, necessarily limit the useful life of the device.
One solution to the above-discussed problems with mechanical mice has been the development of mice using an optical navigation system. These optical mice have become very popular because they provide a better pointing accuracy and are less susceptible to malfunction due to accumulation of dirt.
The dominant technology used today for optical mice relies on a light sources, such as a light emitting diode (LED), illuminating a surface at or near grazing incidence, a two-dimensional (2D) CMOS (complimentary metal-oxide-semiconductor) detector which captures the resultant images, and signal processing unit that correlates thousands of features or points in successive images to determine the direction, distance and speed the mouse has been moved. This technology provides high accuracy but suffers from a complex design and relatively high image processing requirements.
As an improvement, the use of a coherent light source, such as a laser, to Illuminate a rough surface creates a complex interference pattern, called speckle, which has several advantages, including efficient laser-based light generation and high contrast images even under illumination at normal incidence. Laser-based light generation has a high electrical-to-light conversion efficiency, and a high directionality that enables a small, efficient illumination footprint tailored to match a footprint of the array of photodiodes. Moreover, speckle patterns allow tracking operation on virtually any rough surfaces (broad surface coverage), while maintaining the maximum contrast even under unfavorable imaging condition, such as being “out-of-focus”.
An alternative approach for measuring linear displacements uses an optical sensor having one-dimensional (1D) arrays of photosensitive elements, such as photodiodes, commonly referred to as a comb-array. The photodiodes within a 1D array may be directly wired in groups to enable analog, parallel processing of the received signals, thereby reducing the signal processing required and facilitating motion detection. For two-dimensional (2D) displacement measurements using this approach, multi-axes linear arrays have been proposed in which two or more 1D arrays are arranged along non-parallel axes.
Although a significant simplification over prior correlation-type optical mice, these 1D comb-array devices have not been wholly satisfactory for a number of reasons. In particular, one drawback of these devices is their limited accuracy along directions that deviate significantly from the 1D array orientations. This is especially a problem where the optical mouse is moved in an off-axis direction causing the speckle pattern or optical image to enter and leave the field of view of the 1D array too quickly before the image has a chance to build-up an unambiguous signal. This deficiency can be partially remedied by increasing the number of axes, but at the price of reducing the simplicity of the linear comb-array approach.
The approach disclosed in U.S. patent application Ser. No. 11/261,316 (“Two-Dimensional Motion Sensor”) avoids the shortcomings of 1D comb detectors while permitting simpler, more efficient signal processing to provide estimates of 2D displacements.
It is highly desirable to further improve optical navigation apparatus and methods of sensing movement using the same.